M.Sc Student | Lustig Sahar |
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Subject | Piezoelectric Devices for Actuation and Engergy Harvesting |

Department | Department of Mechanical Engineering |

Supervisor | Professor David Elata |

Full Thesis text |

Piezoelectric transducers have great potential in micro actuators and sensors. The large transduction efficiency of piezoelectric structures results from the strong coupling between the mechanical and electrical domains. However, due to this coupling, the governing equations are rather complex, and analysis of practical systems is often challenging.

In this research, we revisit the definition of the piezoelectric coupling factor, and show that the traditional definitions include an inconsistency. This factor measures the amount of energy that may be converted (in quasi static states) to the mechanical/electrical domain, when the piezoelectric structure is subjected to an input in the electrical/mechanical domain (and vice versa). We provide a new insight and show that the coupling factor is not simply a constant that characterizes the material and the boundary conditions on the structure (as is often assumed), but rather it also depends on loading.

Piezoelectric vibrating energy harvesters (PVEH) are emerging as power supplies for autonomous sensor systems and internet of things (IoT) applications. Much effort has been invested in modelling such systems, and in this field as well, there are a few inconsistencies. Specifically there is an ongoing debate whether PVEH should be modeled as current sources or as voltage sources. We suggest that the correct modeling approach is neither, and we present a simple evolution equation for computing the voltage and current in energy harvesters. This evolution equation depends on the specific electric load on the system (e.g. instantaneous heating of a resistor or charging up a battery for later use). We present the analysis of several PVEH systems, and demonstrate the predictive quality of our modeling by comparison to published experimental data.

Piezoelectric single crystals are mechanically more robust relative to poly-crystalline solids, and this suggests that they may have advantages as construction materials for actuators. However, since the poling process of single crystals is more complex than that of poly-crystalline materials, the geometry of the actuator (i.e. electrodes shape and material cuts) is of great significance. As an example, we consider two material cuts of single crystal quartz, as a construction material for twisting-beam and bending-beam actuators. Quartz has a unique material symmetry that leads to no piezoelectric affect in the poling direction, and this unique property affects the optimal configuration of electrodes.

In this work, we also consider single crystal PMN PT as a construction material for a twisting-beam actuator. The design of this actuator was motivated by a poly-crystalline PZT beam, which is coated with rotated interdigitated electrodes (IDEs). The piezoelectric coefficients of PMN PT are higher than of PZT, so we expect to obtain larger torsion angles, but since it is single crystal, its poling process and actuation are more complex. The slanted IDEs are used for both the poling process and for actuation. Using finite elements simulations, the PMN PT beam is found to exhibit a two-fold larger torsion angle, relative to a PZT beam with the same geometry.